Proposed new emissions standards would limit carbon dioxide produced by new power plants, which would probably prohibit construction of any coal-fired facilities.

WASHINGTON — Taking aim at the gases that the vast majority of scientists say are the main contributor to climate change, the Obama administration proposed rules limiting carbon dioxide emissions from new power plants, a move that could essentially bar new coal-fired electric generation facilities.

Tuesday's announcement by the Environmental Protection Agency signaled the administration's willingness to weigh in on politically sensitive environmental issues, even if its decisions court controversy in an election year.

That cheered environmentalists, who had become increasingly concerned in recent months by delays of key EPA rules and President Obama's trumpeting of oil drilling in response to criticism about high gasoline prices. They worried that the carbon rule would be shelved.

By proposing the power plant rules and pressing forward with fuel economy standards for new cars and trucks, the administration has moved to cut pollution from the two largest domestic sources of greenhouse gases. Power plants, which are responsible for 40% of the nation's output of carbon dioxide, are the single greatest stationary source of such emissions.

"I think the administration releasing a proposed regulation for greenhouse gases for new plants is as strong a signal that anyone can ask for about how seriously they are addressing the threat of climate change," said Megan Ceronsky, an attorney for the Environmental Defense Fund.

The proposed emissions standards are for all new plants, including ones powered by abundant and cheap natural gas, but would hit hardest coal-fired facilities, which would face substantial — perhaps insurmountable — technological and financial obstacles in complying with the limits.

"What this essentially says is we will never be building dirty old coal plants ever again," said Michael Brune, executive director of the Sierra Club, one of the litigants in the lawsuit that led to development of the new rules. "The dominant power source of the 19th and 20th centuries won't be the same again."

The rules aren't final, and could be changed by a future Republican administration. Still, major business groups, especially those that benefit from cheap coal-fired power, were harshly critical.

"Requiring coal-based power plants to meet an emissions standard based on natural gas technology is a policy overtly calculated to destroy a significant portion of America's electricity supply," said Hal Quinn, chief executive of the National Mining Assn., whose members include coal companies. "This proposal is the latest convoy in EPA's regulatory train wreck that is rolling across America, crushing jobs and arresting our economic recovery at every stop."

The regulations would apply only to new power plants, not modifications of existing facilities, the standards for which are expected later. The proposed rules would require new plants to emit a maximum of 1,000 pounds of carbon dioxide per megawatt hour. The newest natural-gas-fired power plants emit about 800 pounds of carbon per megawatt hour. New coal plants emit between 1,600 and 1,900 pounds per megawatt hour.

"The agency's action establishes a logical and modest standard for new electric power plants and provides the industry with much-needed regulatory certainty," said Ralph Izzo, chief executive of the utility-holding company Public Service Energy Group, headquartered in New Jersey. "The EPA provides a framework for the industry to confront this problem in a cost-effective manner."

The proposed regulations bolster a years-long trend in which the industry built new, cheaper natural gas plants to replace aging coal-fired generation. Today, there are few new coal plants on the drawing boards.

Moreover, several states, including California, have adopted their own rules to limit carbon emissions associated with their electricity consumption, including power they import from out of state.

"We're not going to build certain plants, but we probably weren't going to build them anyway," said Howard Herzog, senior research engineer for the Massachusetts Institute of Technology Energy Initiative.

Coal accounts for about 50% of the country's power generation. Its popularity has traditionally stemmed from its abundance and low cost.

The new rules could raise the price of coal, making it less attractive to utilities and ratepayers. To slash their carbon dioxide emissions, new coal plants would have to install technology that captures greenhouse gases and stores them in wells or abandoned mines. Currently, there is no large-scale use of carbon capture and storage technology at U.S. power plants, according to MIT.

Cost is the stumbling block, Herzog said. As long as natural gas remains inexpensive, there will be little incentive in the near term to build new, pricier coal plants under the proposed carbon rules.

Yet the move to natural gas also may make prices more volatile. "We've historically had cheaper electricity," Herzog said, "and there's a stability there that in the long term we may not have."

It is unclear how many proposed plants might be affected by the pending rule. About 24 plants have gotten permits or are under construction, according to the Energy Department — and they probably would not be affected by the rule. Another 24 are in early stages of development.

The rules will be open for comment for the next 60 days, but the EPA has not set a date to issue the final rules.